スギの二次師部構成要素の細胞壁構造

Abstract

スギの二次師部の構成要素である師部繊維, 師細胞, 柔細胞および石細胞の各二次壁について, 透過型, 走査型電子顕微鏡, 偏光顕微鏡を使って観察し, 次の結果を得た。(1) 師部繊維, 師細胞および軸方向柔細胞はそれぞれ接線方向に列状に並び, その配列には, 師細胞-師部繊維-師細胞-軸方向柔細胞の順の小週期と, 幾つかの小周期からなる大周期とが認められた。(2) 師部繊維には厚壁で木化が均一に進んでいるもの (Type-1), 厚壁であるが内腔側が未木化のもの (Type-2) および薄壁のもの (Type-3) とが認められた。それらの二次壁はいずれも内層・外層の二層からなっていた。Type-1および2の繊維のフィブリル傾角を偏光顕微鏡で測定したところ, 内層は約45度, 外層は約60度であり, 両者のフィブリルの傾斜方向は互いに反対であった。壁の薄いType-3の繊維では, 透過型電子顕微鏡で観察したところ, 内層・外層のフィブリル配向は, それぞれ厚壁の繊維のものと同じ配向を示した。Type-3の繊維では木化は外層の外側のみで認められた。(3) 師細胞の二次壁は木化せず, 多くのラメラからなっていた。師細胞の側壁に存在する師域は10個前後のsubunitから構成され, 1個のsubunitはさらに数個の師孔からなっていた。(4) 軸方向柔細胞の二次壁は, 師細胞と同じく, 多くのラメラからなっていた。柔細胞の側壁のpitは二・三の領域からなり, その領域間にはミクロフィブリルの束が仕切として存在していた。(5) 石細胞は厚壁の師部繊維の列にあらわれ, その二次壁は多くの厚い層からなっていた。

The authors examined the secondary wall structure of fibers, sieve cells, axial parenchyma cells and stone cells (sclereids) in the secondary phloem of Cryptomeria japonica D. DON. 1) Elements in the phloem were arranged respectively in uniseriate tangential bands (Photo. 1) and radial cell cycles of two kinds were found in them, i.e. the short cycle composed of four bands, sieve cell--fiber--sieve cell--parenchyma cell and the long cycle composed of several short cycles (Fig. 2). 2) Some of the fiber bands had thick-walled fibers and the others had thin-walled ones. The secondary wall in both fibers was composed of two layers, i.e. the outer layer and the inner layer (Photo. 5, 6, 7). 3) In the thick-walled fibers, the fibril angle of the inner layer measured by polarizing microscope was about 45°to the cell axis, and that of the outer layer was about 60°. In the outer layer fibrils inclined in the opposite direction to those in the inner layer (Photo. 9). Examination with phloroglucinol-hydrochloric acid staining (Photo. 3) in light microscopy, and potassium permanganate staining (Photo. 5, 6) in electron microscopy showed that the outer layer was strongly lignified but the inner layer had considerable variation in lignification among the tangential fiber bands. In the less lignified fiber bands, the lumen side zone of the inner layer was not lignified. 4) Electron microscopy also revealed that the fibrillar orientation of the two layers in thin-walled fibers was similar to that in thick-walled ones (Photo. 10, 11), and lignification of the wall, investigated by the delignification method of ultrathin sections, was restricted to the outer region of the outer layer (Photo. 8). 5) The secondary wall of the sieve cell was composed of many lamellae (Photo. 12) and was not lignified. The sieve area consisted of subunits and each subunit possessed several sieve pores. 6) The secondary wall of the axial parenchyma cell was also composed of many lamellae and was not lignified. The pit on the lateral wall was composed of two or three areas, and between these areas a bundle of microfibrils existed as a boundary. 7) Stone cells sometimes occurred in the thick-wall fiber bands. Their secondary wall was composed of many thick lamellae (Photo. 16). The cell wall structures of the phloem elements mentioned above were diagrammatically represented in Fig. 4.